This application claims the benefit of Korean Patent Application No. 2000-00398, filed on Jan. 6, 2000, under 35 U.S.C. xc2xa7119, the entirety of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a transflective LCD device.
2. Description of the Related Art
In general, a transflective liquid crystal display (LCD) device selectively acts as a transmissive LCD device and as a reflective LCD device. Due to the fact that a transflective LCD device can make use of both internal and external light sources, it can operate in bright ambient light and with low power consumption.
FIG. 1 shows a typical thin film transistor liquid crystal display (TFT-LCD) device 11. The TFT-LCD device 11 includes upper and lower substrates 15 and 21 with an interposed liquid crystal 23. The upper and lower substrates 15 and 21 are sometimes respectively referred to as a color filter substrate and an array substrate.
On a surface facing the lower substrate 21, the upper substrate 15 includes a black matrix 16 and a color filter layer 17. The color filter layer 17 includes a matrix array of red (R), green (G), and blue (B) color filters that are formed such that each color filter is bordered by the black matrix 16. The upper substrate 15 also includes a common electrode 13 over the color filter layer 17 and over the black matrix 16.
On a surface facing the upper substrate 21, the lower substrate 21 includes an array of thin film transistors (one being designated as TFT xe2x80x9cTxe2x80x9d in FIG. 1) that act as switching devices. The array of thin film transistors is formed to correspond with the matrix of color filters. A plurality of crossing gate and data lines 25 and 27 are positioned such that a TFT is located near each crossing of the gate and data lines 25 and 27. The lower substrate 21 also includes a plurality of pixel electrodes 19, each in an area defined between the gate and data lines 25 and 27. Such areas are often referred to as pixel regions xe2x80x9cP.xe2x80x9d
Each pixel electrode 19 includes a transparent portion 19a and a reflective portion 19b. The transparent portion 19a is usually formed from a transparent conductive material having good light transmissivity, for example, indium-tin-oxide (ITO). Alternatively, the transparent portion 19a can be a hole. However, in FIG. 1 a transparent conductive material is employed for the transparent portion 19a. Moreover, a conductive metallic material having a superior light reflectivity is used for the reflective portion 19b. 
FIG. 2, a cross-sectional depiction of a transflective LCD device 57, helps illustrate the operation of such devices. As shown in FIG. 2, the transflective LCD device 57 includes lower and upper substrates 53 and 43 and an interposed liquid crystal layer 56. The upper substrate 43 includes a common electrode 33. The lower substrate 53 includes transparent and reflective electrodes 51 and 49 that act as a pixel electrode. The transflective LCD device 57 also includes a backlight device 41.
The reflective electrode 49, made of a conductive material having a good reflectivity, surrounds the transparent electrode 51. The transparent electrode 51 transmits light xe2x80x9cAxe2x80x9d irradiated from the backlight device 41, while the reflective electrode 49 reflects the ambient light xe2x80x9cB.xe2x80x9d
The transflective LCD device 57 is operable in both a reflective mode and a transmissive mode. In the reflective mode, the ambient light xe2x80x9cBxe2x80x9d passes through the upper substrate 43 and reflects from the reflective electrode 49 back toward the upper substrate 43. With an electrical signal applied between the common electrode 33 and the pixel electrode (reflective electrode 49 and transparent electrode 51) by the switching element xe2x80x9cTxe2x80x9d (see FIG. 1), the phase of the liquid crystal layer 56 changes. Thus, the light xe2x80x9cBxe2x80x9d passing through the liquid crystal layer 56 is colored by the color filter 17 (see FIG. 1) and is displayed as a colored pixel.
In the transmissive mode, light xe2x80x9cAxe2x80x9d from the backlight device 41 passes through the transparent electrode 51. With an electrical signal applied between the common electrode 33 and to the pixel electrode (reflective electrode 49 and transparent electrode 51) by the switching element xe2x80x9cTxe2x80x9d (see FIG. 1), the phase of the liquid crystal layer 56 changes. Thus, the light xe2x80x9cAxe2x80x9d passing through the liquid crystal layer 56 is colored by the color filter 17 (see FIG. 1) and is displayed as a colored pixel.
As described above, since the transflective LCD device 57 has both a transmissive mode and a reflective mode, the transflective LCD device can be used anytime, day or night. It also has the advantage of being battery operable for a long time because of its low power drain. However, a significant amount of light from the backlight device is lost in the transmissive mode.
FIG. 3 is a cross-sectional depiction of another conventional transflective LCD device 58. As shown, an upper retardation film 42 and an upper polarizer 45 are formed on an upper substrate 43. A lower retardation film 50 and a lower polarizer 47 are formed under a lower substrate 53. Moreover, a liquid crystal 55 is interposed between the upper substrate 43 and the lower substrate 53. On the inner surface of the lower substrate 53 are reflective electrodes 49 and transparent electrodes 51 (only one of each is shown in FIG. 3). The lower substrate 53 also includes gate and data lines 25 and 27 that define pixel regions xe2x80x9cPxe2x80x9d (reference FIG. 1). The transparent electrode 51 and the reflective electrode 49 that form the pixel electrode are in a pixel region xe2x80x9cP.xe2x80x9d
The LCD panel 58 is divided into an open region xe2x80x9cExe2x80x9d and a closed region xe2x80x9cF,xe2x80x9d depending on whether light xe2x80x9cCxe2x80x9d and xe2x80x9cDxe2x80x9d from a backlight device 41 passes through the LCD panel 58. The closed region xe2x80x9cFxe2x80x9d is associated with an opaque metallic material, including the reflective electrode 49 and the gate lines 25 and data lines 27 (see FIG. 1). The open region xe2x80x9cExe2x80x9d is associated with the transparent electrode 51.
In the transmissive mode of the LCD panel 58, the light xe2x80x9cDxe2x80x9d passes through the transparent electrode 51 into the liquid crystal layer 55. Most of the light xe2x80x9cCxe2x80x9d is absorbed by the lower polarizer 47 after being reflected by the reflective electrode 49. However, a small amount of the light xe2x80x9cCxe2x80x9d does pass through the liquid crystal 55.
FIG. 4 shows the states of the light from the backlight device as that light passes through the LCD panel 58. The light from the lower polarizer 47 is linearly polarized. The lower polarizer 47 absorbs much of its incident light, except that part that is parallel with the transmitting axis of the lower polarizer 47. Therefore, lower polarizer 47 significantly reduces the light density of its incident light.
The linearly polarized light that passes through the lower polarizer 47 is then changed into left-circularly polarized light by the retardation film 50, which has a phase difference of xcex/4. Some of the left-circularly polarized light passes through the liquid crystal 55 associated with the open portion xe2x80x9cExe2x80x9d (see FIG. 3). The remainder of the left-circularly polarized light is reflected by the reflective electrode 49 (see FIG. 3) and is changed into right-circularly polarized light due to a mirror effect. The right-circularly polarized light then enters into the retardation film 50 again, where it is converted into linearly polarized light having a phase difference angle of xcex/4.
Moreover, when the linearly polarized light from the retardation film 50 enters the lower polarizer 47, the phase of the linearly polarized light is perpendicular to the transmitting axis of the lower polarizer 47. Therefore, the lower polarizer 47 absorbs most of that light.
As a result, the conventional transflective LCD device suffers a serious decrease in brightness because the closed portion xe2x80x9cFxe2x80x9d (see FIG. 3) associated with the reflective electrode 49 and with the gate and data lines causes a significant amount of light absorption.
Accordingly, the present invention is directed to a transflective liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the invention is to provide a method of fabricating a transflective LCD device (as well as the transflective LCD device itself) that increases the brightness by reducing or preventing light from being absorbed by a lower polarizer.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a transflective liquid crystal display device includes a first polarizer on a first retardation film, which itself is on a first substrate. A color filter is under the first substrate, and a transparent common electrode is below the color filter. A lower substrate assembly is located below the transparent common electrode. A liquid crystal layer is interposed between the transparent common electrode and the lower substrate assembly.
The lower substrate assembly includes a reflective electrode having a light passing portion. The reflective electrode is adjacent a second polarizer made of a cholesteric liquid crystal. A second retardation film is disposed between the reflective electrode and the second polarizer. The second retardation film includes a first portion, which corresponds in size to the light passing portion of the reflective electrode, that transmits light without a polarization change, and a second portion that transmits light with a phase difference of xcex/4. A CLC color filter is disposed between the second polarizer and the liquid crystal layer. The lower substrate assembly further includes a backlight device having a reflective surface. The second polarizer is disposed between the backlight device and the reflective electrode. The lower substrate assembly also beneficially includes a transparent substrate.
Beneficially, the CLC color filter can be a CLC color filter layer that is disposed between the CLC polarizer and the second retardation film. Another beneficial location for the CLC color filter is in the light passing portion of the reflective electrode. Another beneficial location for the CLC color filer is in the first portion of the second retardation film. Alternatively, the CLC color filter can be located both in the first portion of the second retardation film and in the light passing portion of the reflective electrode.
In accordance with the purpose of the invention, in another aspect the principles of the present invention provide for a transflective liquid crystal display device, including: a first polarizer; a first retardation film under the first polarizer; a first substrate under the first retardation film; a color filter under the first substrate; a reflective electrode having a transparent portion, wherein the transparent portion is filled up with a CLC color filter; a liquid crystal layer interposed between the color filter and the reflective electrode; a second retardation film formed under the reflective electrode, wherein the second retardation film has a first portion that is filled up with the CLC color filter and corresponds in size to the transparent portion of the reflective electrode, also wherein the second retardation film has a second portion that transmits the light with a phase difference xcex/4; a second polarizer made of a cholesteric liquid crystal and formed below the second retardation film; a second substrate formed between the second polarizer and the reflective electrode; and a backlight device arranged below the second substrate and irradiating the light to the second polarizer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.